Even at base camp, at 5,300m, it can be tough coping with the thinner air. Many people develop headaches and painful hacking coughs. They can find it harder to sleep and eat.

Meanwhile, specialist climbing Sherpas - from the Everest region - have already begun preparing the route, fixing ladder bridges across crevasses on the first section through the treacherous Khumbu glacier or icefall.

They are known as the "ice doctors" and they re-check and repair the route every day in case of avalanches or movements on the glacier. Most expeditions depend on Sherpa support.

Further up, they fix safety ropes on the more difficult sections of the mountain.

It is often six weeks or more after arriving at base camp before climbers actually go for the summit.

They spend that time making a series of incremental ascents to get used to the thin air, using a "climb high, sleep low" approach - progressively going higher but then descending again to rest.

Route plan

The route up is like this: first, climbers have to negotiate the crevasses and ice walls of the Khumbu icefall.

EVEREST FACTS AND FIGURES

Height: 8,848 - 8,850m (29,029 - 29,035 feet)

First ascent: 29 May, 1953, Edmund Hillary/Tenzing Norgay

Number of ascents up to 1988: less than 200

Number of ascents up to end of 2008 climbing season: 4,109 by 2,700 individuals

Most people on the summit in a single day: 116, on 22 May, 2003

Oldest summiteer: 76-year-old Nepalese man, Min Bahadur Sherchan

On their final summit push, they usually bypass Camp 1 and go straight to Camp 2 or Advanced Base Camp at about 6,400m, bypassing Camp 1.

Next day, there is a steep climb up the face of the neighbouring peak of Lhotse. Spiked steel crampons are essential on the sometimes rock-hard ice.

They stay the night at Camp 3 about halfway up the face before tackling the rockier sections of the Yellow Band and then the Geneva Spur.

This leads to Everest's southern shoulder, the South Col, and Camp 4 at just under 8,000 metres. This is the launching point for the summit almost a kilometre above.

Climbers are now in the "Death Zone", so-named because there's so little oxygen in the air that nothing - plant or animal - can live up there for long.

Scarce oxygen

Using bottled oxygen helps, but does not cancel out the effects of being that high.

The body is literally starting to break down, using up its stores. So it is a race against time.

Thousands of people have now climbed Everest [Pic: Mark Georgiou]

On the summit, there is less than a third of the amount of oxygen available as at sea level.

Typically, climbers start out for the summit late at night, aiming to be on top the following morning with enough time to get all the way back to the South Col in daylight.

For this reason, team leaders impose turnaround times. If their climbers are not close enough to the summit by that time - often about 1pm - they have to descend.

It does not always work out like that. So near, people keep on going but then don't have the energy to get down again.

Experienced mountaineers will often say that Everest is not technically difficult to climb by the South Col route, compared with other Himalayan peaks.

But at this altitude, battling with hypoxia or oxygen starvation, each step is a decision.

'The longest walk'

The thin air plays with your mind. Many die here because they are not able to think straight.

Sometimes the only landmark ahead is the corpse of a dead climber.

There is one last obstacle before the top - a rocky outcrop called the Hillary Step, named after Sir Edmund Hillary, the first man to climb Everest with Sherpa Tenzing Norgay.

After that, the summit is about 30 or 40 minutes away, up an undulating snow slope. But "it's the longest walk in the world," says guide Kenton Cool. "Even when you finally see the summit flags ahead, that walk seems never-ending."

Then it is all the way down again, many climbers sleeping a night at the South Col before returning to base camp next day.

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